When / where is the form_show event executed?
Open a form in the designer, and scroll to the bottom of the Properties window. That will show you the available events for the selected component. (If you’ve just opened the form and not clicked anything else, or if you click on the background, the selected component is the form itself). Click the blue arrow next to the show event to run some code when that event fires.
The show event on any component fires when the component is added to the webpage.